Vircell assays for detection of antibodies against Legionella pneumophila.

We read with interest the study by Diederen et al. (1), in which the serological assays from Vircell were evaluated. We found that this work underlines the importance of immunoglobulin M (IgM) detection in the diagnosis of legionellosis previously described by our group (3). Yet, we would like to point out some disagreements with some of the information given in the aforementioned study. The authors used enzyme-linked immunosorbent assay (ELISA) as a reference technique for calculating the sensitivity and specificity of Vircell’s assays. We consider that the references cited in the paper do not show enough validation to the two commercial tests for either of them to be considered a “gold standard”: in the cited studies, sensitivities of 78% and 74%, respectively, were calculated for the Serion IgG and IgM ELISAs. Besides, in a previous comparison of the immunofluorescence assay (IFA) and ELISA IgG tests of the same commercial brands (2), completely different results were obtained: 67% out of 117 patients from a Legionella outbreak were serologically diagnosed by the Vircell IFA against 40% diagnosed by the Serion IgG ELISA. Therefore, only samples from patients with a diagnosis confirmed by other direct methods should have been included in calculations of the sensitivities of the assays for both commercial brands; the remaining samples should have been included only for the purpose of determining the diagnostic concordance between tests. Otherwise, additional serological tests (either commercial or in-house) should have been included to elucidate discrepant results. In any case, the data for the second manufacturer should have been given in Table 1 together with the clinical data (i.e., sensitivity and specificity values) of Vircell’s tests. It is obvious that if one single test is considered a reference method, any other assay evaluated against it will render worse results. The specificities for the different tests (given in Tables 2 and 3) were calculated taking into account the results from 179 samples, 129 of which belonged to patients with legionellosis. That is to say, if the assay of the other manufacturer yielded a negative result, then a positive result in the Vircell assay was considered to be a false positive for the purpose of specificity calculation. In our opinion, this method of calculating falsepositive results is not correct. Only samples from populations free of Legionnaires’ disease should have been included for the evaluation of the specificity of the tests. The confidence intervals also should have been included in the statistical analysis of the results. Finally, we found the results shown in Tables 2 and 3 difficult to interpret. It is not clear which of the data in the table correspond to the Vircell assays and which to Serion assays. If the Serion assay results are given in rows, the sensitivity and specificity values for Vircell’s tests are not well calculated. As an example, the respective sensitivities for the Vircell IgM, IgG, and IgG-plus-IgM ELISAs should be 97.3%, 55.2%, and 96.0% instead of the 82.0%, 88.9% and 90.0% shown in the footnote of Table 3. On the other hand, if the Serion assay results are given in the columns, as the sensitivities and specificities shown in the tables would indicate, then either Tables 2 and 3 report different numbers of total positive and negative samples scored by the Serion IgM and IgG ELISA assays (for example, 61 positive IgM samples in Table 2 versus 91 in Table 3) or Table 3 reports different numbers depending on the column (compare the total number of samples positive by IgM ELISA with the number positive by IgM-plus-IgG ELISA [91 versus 82] and the total number of samples negative by IgM ELISA with the number negative by IgM-plus-IgG ELISA) [82 versus 89]). In that sense, it would be desirable for the authors to clarify these data.